U.S. patent number 4,745,401 [Application Number 06/773,593] was granted by the patent office on 1988-05-17 for rf reactivatable marker for electronic article surveillance system.
This patent grant is currently assigned to Minnesota Mining and Manufacturing Company. Invention is credited to Samuel Montean.
United States Patent |
4,745,401 |
Montean |
May 17, 1988 |
RF reactivatable marker for electronic article surveillance
system
Abstract
A marker for use in radio frequency electronic article
surveillance systems where the marker contains an
inductive-capacitive resonant circuit and is made reversibly
deactivatable and reactivatable by the addition of a piece of
magnetic material and means, such as a piece of permanently
magnetizable material, for biasing the first material to prevent
alternating fields induced therein from changing the magnetic state
of that material, thereby preventing hysteresis losses from causing
a lowering of the Q of the resonant circuit below the point of
detection.
Inventors: |
Montean; Samuel (Blaine,
MN) |
Assignee: |
Minnesota Mining and Manufacturing
Company (St. Paul, MN)
|
Family
ID: |
25098753 |
Appl.
No.: |
06/773,593 |
Filed: |
September 9, 1985 |
Current U.S.
Class: |
340/572.3;
333/175; 333/185; 342/51; 343/788; 343/895; 340/572.5;
257/E27.114 |
Current CPC
Class: |
G08B
13/2457 (20130101); G01V 15/00 (20130101); G08B
13/2437 (20130101); G08B 13/2442 (20130101); G08B
13/242 (20130101); H01L 27/01 (20130101) |
Current International
Class: |
G01V
15/00 (20060101); H01L 27/01 (20060101); G08B
13/24 (20060101); G08B 013/18 () |
Field of
Search: |
;343/787,788,895
;333/185,175 ;340/572,552,553 ;342/27,42,51 ;336/200,232 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2119603 |
|
Nov 1983 |
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GB |
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2121652 |
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Dec 1983 |
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GB |
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2148668 |
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May 1985 |
|
GB |
|
Other References
Snyder, Albers-Schoenberg, and Goldsmith; "Magnetic Ferrites-Core
Materials for High Frequencies"; Electrical Manufacturing; Dec.
1949 Issue; pp. 86-91..
|
Primary Examiner: LaRoche; Eugene R.
Assistant Examiner: Lee; Benny T.
Attorney, Agent or Firm: Sell; Donald M. Smith; James A.
Barte; William B.
Claims
I claim:
1. In an electronic article surveillance system having a receiver
for detecting oscillations from a tuned resonant circuit; a
resensitizable marker comprising therein, an inductive-capacitive
resonant circuit, a first magnetic material, and means for
controlling a magnetic circuit formed by said first material,
wherein said resonant circuit includes an inductive component
having one or more turns, said first magnetic material includes at
least one piece of magnetic material of a first type having a low
coercive force and high permeability and substantially encloses
only a portion of said one or more turns of said inductive
component, and said means for controlling the magnetic circuit
formed by said material of the first type causes the resonant
circuit to alternately be in a first sensitized state and a second,
desensitized state.
2. A marker according to claim 1, wherein said resonant circuit
exhibits a Q-factor greater than a predetermined value when in a
sensitized state and less than said predetermined value when in a
desensitized state, and wherein said magnetic circuit controlling
means comprises means for alternately establishing a said magnetic
circuit having states of low and high efficiency, said high
efficiency state being such as to cause sufficient hysteresis
losses therein to reduce said Q-factor below said predetermined
value such that said marker is in a desensitized state.
3. A marker according to claim 1, wherein the first type of
magnetic material is permalloy.
4. A marker according to claim 1, wherein said magnetic circuit
controlling means comprises a member formed of a second type of
permanently magnetizable magnetic material adjacent said first type
of magnetic material.
5. A marker according to claim 4, wherein said second type of
magnetic material is vicalloy.
6. A marker according to claim 4, wherein the first type of
magnetic material comprises a ferromagnetic material which can be
magnetized to saturation by an applied magnetic field not exceeding
several oersteds and wherein said second type of magnetic material
comprises a material which in a magnetized state exhibits an
external magnetic field on the magnetic material of the first type
of sufficient intensity to saturate the magnetic material of the
first type.
7. A marker according to claim 1, wherein said inductive component
comprises at least one loop and said magnetic material of the first
type is wrapped around a portion of one or more turns of the
loop.
8. A marker according to claim 1, wherein said magnetic material of
the first type is folded back on itself so as to only partially
encompass a portion of one or more turns of said inductive
component.
9. A marker according to claim 1, wherein at least two portions of
said magnetic material are positioned on opposite sides of one or
more turns of the inductive component and extends beyond said
inductive component, the extending portions being positioned
opposite to each other so as to be magnetically coupled together to
thereby complete said magnetic circuit.
10. A marker according to claim 4, wherein said permanently
magnetizable material comprises at least one sheet of such material
extending over a significant portion of the magnetic material of
the first type.
11. A marker accordding to claim 10, comprising at least two sheets
of permanently magnetizable material, one sheet being positioned on
each side and adjacent to a section of said magnetic material.
12. A marker according to claim 1, comprising a sheet of a said
first type of magnetic material folded back on itself so as to only
partially encompass a portion of one or more turns of said
inductive component, and further extending therebeyond such that
extending portions of said sheet are juxtaposed so as to be
magnetically coupled, and further comprising a section of a
permanently magnetizable material removably positionable between
said extending portions.
13. An electronic article surveillance system comprising
(a) means for generating within an interrogation zone a radio
frequency electromagnetic field
(b) marker means having therein a tuned, inductive-capacitive
circuit resonant at a predetermined frequency and having a Q-factor
associated therewith in excess of a predetermined value such that
when said marker means is in said zone, said circuit responds to
said field by absorbing and radiating energy at said frequency
and
(c) receiver means for receiving energy radiated from said marker
means when the effective Q-factor therein is in excess of said
predetermined value and for activating an output in response
thereto;
wherein said marker means further comprises therein, (i) the
resonant circuit having a Q-factor in a sensitized state greater
than said predetermined value and a Q-factor in a desensitized
state less than said predetermined value, (ii) at least one piece
of magnetic material substantially enclosing at least a portion of
one or more turns of said inductive component and (iii) means for
controlling a magnetic circuit formed by said material to
alternately establish a low efficiency and a high efficiency
condition in said magnetic circuit, said high efficiency condition
being such as to cause sufficient hysteresis losses therein to
reduce said Q-factor below said predetermined value such that said
marker is then in a desensitized state.
14. A system according to claim 13, wherein said means for
controlling the magnetic circuit comprises a permanently
magnetizable member adjacent said magnetic material.
15. In an electronic article surveillance system having a receiver
for detecting oscillations from a tuned resonant circuit; a
resensitizable marker comprising an inductive-capacitive resonant
circuit including therein, an inductive component having one or
more turns, at least one sheet-like piece of magnetic material of a
first type haivng a low coercive force and high permeability and
folded back on itself so as to only partially enclose a portion of
said one or more turns of said inductive component and further
extending therebeyond such that extending portions of said sheets
are juxtaposed so as to be magnetically coupled, and means
comprising at least a section of permanently magnetizable material
removably positionable between said extending portions for
controlling the magnetic circuit formed by said material of the
first type to thereby cause the resonant circuit to alternately be
in a first sensitized state and a second, desensitized state.
16. In an electronic article surveillance system having a receiver
for detecting oscillations from a tuned resonant circuit, a
resensitizable marker comprising an inductive-capacitive resonant
circuit including therein, an inductive component having one or
more turns, at least one piece of magnetic material of a first type
having a low coercive force and high permeability and substantially
enclosing only a portion of said one or more turns of said
inductive component, and means comprising a member formed of a
second type of permanently magnetizable magnetic material extending
over a significant portion of said first type of magnetic material
for controlling the magnetic circuit formed by said material of the
first type to thereby cause the resonant circuit to alternately be
in a first sensitized state and a second, desensitized state.
Description
FIELD OF THE INVENTION
This invention relates to electronic article surveillance systems
of the type in which a marker containing an inductive-capacitive
circuit resonant at at least one frequency, is utilized in
conjunction with radio frequency electromagnetic fields operating
in tne range or the resonant frequency in order to cause
oscillations in the circuit which are remotely detected. Such
systems are particularly used to thwart shoplifting and to
otherwise detect the passage of controlled objects through a
surveillance zone.
BACKGROUND OF THE INVENTION
In surveillance systems of the type referred to above, the markers
have generally been single status, i.e., they have not been able to
be reused, i.e., repeatedly, deactivated and subsequently
reactivated. Such a limitation has, for the most part, restricted
the use of such systems to applications in which the marker is
physically removed from the object when detection thereof is no
longer desired, such as at the point of sale. It has also been
proposed to physically and irreparably destroy or alter the
resonant circuit within the marker, such as by a fusible link which
is melted so as to open a portion of the circuit. (See Lichtblau,
U.S. Pat. No. 3,810,147). Such a scheme has not found commercial
acceptability, possibly due to the expense of the markers, which
become useless after the circuit is thus irreparably destroyed or
altered.
As the resonant circuits used in the markers are readily affected
by a conductive sheet placed in close proximity, it is also known
to selectively deactivate such markers by positioning such a sheet
i.e., an aluminum foil, next to the marker. For example, such a
marker could be concealed within the UPC label or beneath the
pocket in a library book into which a "check-out" card is to be
inserted. A conductive foil concealed within the check-out card or
within a separate paste-on label would then be provided. Obviously,
that scheme requires separate deactivatable components, and may be
impractical for use in many situations.
One suggestion for providing a deactivatable/reactivatable RF
marker is set forth in U.S. Pat. No. 3,493,955 (A. J. Minasy). In
that patent, it is proposed that a small, non-conductive but
magnetizable element such as a ferrite can be positioned on or
close to a coil in the marker. It is stated in that patent that
normally the ferrite will be in its non-magnetic state and will
have no effect on the operation of the marker, but that when it is
desired to deactivate the marker, the ferrite be brought to its
magnetic state, thereby interfering with the electromagnetic fields
of radio waves in the vicinity, and hence effectively preventing
operation of the device. A large electromagnet is proposed to be
used to switch the ferrite back and forth between its magnetic and
non-magnetic state. Such a concept is not known to have ever been
successfully utilized.
U.S. Pat. No. 4,063,229 (Welsh et al) depicts yet another type of
EAS system wherein a marker or tag containing an electrically
non-linear element such as a diode is used to generate harmonics of
a transmitted microwave signal, typically at 100 or 915 MHz.
Harmonics, typically second order at 200 or 1830 MHz, are then
detected. Such a marker does not have an intrinsic resonant
frequency, and the diode is provided with an antenna tuned to the
transmitted frequency to enhance the absorption of energy and the
transmission of harmonic radiation. That patent (Col. 18, line
47ff, and FIGS. 10 and 11) also suggests that the tags can be made
deactivatable by providing layers of two ferrites adjacent inner
and outer antenna loops joined together via a non-linear capacitor
such as a reverse biased diode. The first ferrite layer (407) is
proposed to be a high retentivity, permanently magnetizable
ferrite, while the second layer (408) is a soft low retentivity
ferrite. The tag is said to be activatable by magnetically
saturating the first layer. Flux from that layer returns through
the second layer which is thereby also saturated, and the
inductances of the antenna loops are, therefore, generally
unaffected. To deactivate the tag, the first ferrite layer is
demagnetized. The second layer then possesses high permeability and
increases the inductance of the loops to about twice their former
value, thus reducing the reaction fields below a detectable
level.
FIGS. 15 and 16 and the accompanying description (Column 20, lines
12-24) of the Welsh '229 patent suggest a deactivatable tag in
which a single tuned loop circuit is used which resonate at the
fundamental system frequency, and in which no non-linear element is
provided. First and second ferrite layers (407 and 408) are
provided as in the embodiments shown in FIGS. 10 and 11. It is
there suggested that such an embodiment could be employed in
applications in which selectivity does not pose a problem because
no articles are present which are sufficiently conductive to
distort the applied fundamental frequency field through creation of
eddy currents. Notwithstanding the reasonable commercial success
enjoyed by such diode containing tags and associated systems
transmitting at microwave frequencies, it is not believed that
deactivatable tags as described in the '229 patent have ever been
found to be practical.
In another, totally different, type of electronic article
surveillance system, magnetically deactivatable and reactivatable
markers have been successfully employed for a number of years. See,
for example, U.S. Pat. No. 3,665,449, Elder & Wright. Such
systems utilize a marker which is itself magnetic, comprising an
elongated strip of low coercive force, high permeability
ferromagnetic material, adjacent to which is positioned at least
one piece of a higher coercive force, permanently magnetizable
material. When the magnetization in such a strip is reversed by a
low frequency alternating magnetic field produced in an
interrogation zone, detectable harmonics of that frequency are
generated. In direct opposite to that suggested by Welsh et al
('229), such a magnetic marker is deactivated by magnetizing the
higher coercive force, permanently magnetizable material. The
magnetized material magnetically biases the low coercive force
material and prevents the magnetization therein from reversing due
to the alternating field present in the interrogation zone, thus
preventing its detection, i.e., deactivating it. The
deactivatability thus provided has greatly contributed to the
significant commercial success enjoyed by such systems over the
past decade. The absence of a practical reversible-deactivation
capability has, on the other hand, appreciably restricted the areas
in which the other systems could be used and has thereby lessened
the commercial success of such systems.
SUMMARY OF THE INVENTION
Despite the advantages presented by the magnetic
deactivation/reactivation principles employed in systems such as
disclosed in U.S. Pat. No. 3,665,449, and despite a long standing
desire to provide deactivatable/reactivatable markers for use in RF
EAS systems, the only previous suggestions of such an RF marker are
those set forth in Minasy '955, and Welsh '229, and those
suggestions have not proven to be feasible. In contrast, the
present invention manifests, for the first time, just such a
deactivable/reactivatable RF marker and indeed utilizes magnetic
principles to accomplish that highly desirable result.
The marker of the present invention is, therefore, for use in an
electronic article surveillance system having a receiver for
detecting oscillations from a sharply tuned resonant circuit when
the Q-factor associated therewith exceeds a predetermined value.
The marker itself comprises such a resonant circuit, the circuit
having a multi-turn inductive component and having a Q-factor
greater than the predetermined value when in a sensitized state and
less than the predetermined value when in a desensitized state.
Typically, as is well known, such a circuit will also include a
capacitive component in order to complete a circuit resonant at at
least a given frequency. The marker further comprises at least one
piece of magnetic material forming a magnetic circuit substantially
enclosing at least a portion of the inductive component and means
for controllably magnetically biasing the magnetic material.
Preferably, the magnetic biasing means comprises a permanently
magnetizable member. Removing the magnetic bias results in a
reduction in the Q-factor below the predetermined value, and
desensitizes the circuit.
In the present invention, it has been found that such a presence of
the piece of magnetic material, by itself, causes the Q-factor to
be decreased from that which would exist were no such material
present, to a level at which the marker is nominally not
detectable. Further, it has been found that when a non-varying
magnetic field is impressed upon the magnetic material no
appreciable reduction in the Q-factor is observed. Such a
non-varying field is conveniently provided by positioning a piece
of permanently magnetizable material adjacent the first
material.
It is generally believed that the observed reduction in the
Q-factor is due to the loading effect which the first magnetic
material has on the inductive component. Since the reduction in the
Q-factor has also been found to be controllable by the application
of a non-varying magnetic field, it appears reasonable to assume
that the loading effect is due to hysteresis losses in the first
magnetic material, which losses are coupled to and directly affect
the Q of the circuit. Hysteresis losses are associated with changes
in the magnetization of the first magnetic material, thus a
reduction in any magnetization change would, in turn reduce
hysteresis losses and would thereby prevent a reduction in the
Q-factor. As noted above, in the present invention, the reduction
in any magnetization change is achieved by magnetically biasing the
first material with a non-varying magnetic field.
For hysteresis losses to occur, however, it is first necessary that
a change in the magnetization of the first magnetic material occur.
The greater changes in the magnetization would cause more
hysteresis losses and hence a greater reduction in the Q-factor. It
would thus be desirable to maximize a controllable magnetization
change. One must, therefore, consider how the magnetization of the
first magnetic material in the proximity of the LC marker circuit
can be caused to change. It will first be recognized that the
alternating electromagnetic fields applied in the interrogation
zones of EAS system induce electric resonant oscillating currents
in the inductive components included in the marker circuit.
Further, it is now appreciated both that the oscillating currents
produce a corresponding localized oscillating electromagnetic field
and that that field can both affect and be affected by magnetic
materials in close proximity. This effect is to alter the
magnetization state of the first magnetic material, and the effect
of the altered magnetization state, and of hysteresis losses
associated therewith, is to extract energy from the field,, i.e.,
to make it appear to be more "lossy", which, effect ultimately
becomes apparent as a reduction in the Q-factor.
In contrast, when the first magnetic material is biased in one
direction, the localized oscillating field has an insufficient
intensity to appreciably alter the magnetic state. Accordingly,
hysteresis losses do not extract appreciable energy. The Q-factor
is thus substantially unaffected by the presence of the magnetic
material and remains in excess of the predetermined value so that
the marker is sensitized, e.g., detectable.
Preferably, as noted above, the magnetic biasing means comprises a
permanently magnetizable member. Such a member is thought to
provide a non-varying external magnetic field in the vicinity of
the first magnetic material which is more intense than that
provided by the localized alternating electromagnetic field arising
from oscillating currents in the inductive component of the marker.
Such a undirectional bias field would effectively saturate the
magnetic material and thereby prevent the alternating field from
substantially altering the magnetic state of the magnetic material.
As the magnetic state is unchanged, hysteresis losses are
minimized, and in the absence of such losses, the effective
Q-factor remains in excess of the predetermined value. The marker
is then sensitized and detectable.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a plan view of one embodiment of a marker of the present
invention;
FIG. 2 is a cross-sectional view of the embodiment shown in FIG. 1,
when along the lines 2--2;
FIG. 3 is a plan view of another embodiment of a marker of the
present invention;
FIG. 4 is a cross-sectional view of the embodiment shown in FIG. 3,
taken along the lines 3--3;
FIGS. 5-7 are further cross-sectional views of other embodiments
which in plan view would be the same or similar to those shown in
FIGS. 1 and 3, but wherein somewhat different layered constructions
are present;
FIG. 8 is a plan view of another embodiment of the marker of the
present invention utilizing a discrete capacitor and a multiple
turn wire wound inductor;
FIG. 9 is a plan view of another embodiment utilizing a printed
circuit construction;
FIG. 10 is a cross-sectional view of the embodiment shown in FIG.
9, taken along the lines 10--10;
FIG. 11 is a plan view of another embodiment of a marker of the
present invention;
FIG. 12 is a partial plan view of a different embodiment of a
marker using individual magnetic pieces over each winding of the
inductive component;
FIG. 13 is a cross-section of the marker shown in FIG. 12 taken
along the line 13--13;
FIG. 14 is a cross-section of a similar embodiment as that shown in
FIGS. 12 and 13;
FIG. 15 is a partial cross-section of another embodiment using a
removable permanently magnetized element; and
FIG. 16 is a pictorial view of a system using the marker of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
EXAMPLE 1
FIG. 1 sets forth a plane view of one preferred embodiment of the
marker of the present invention, a portion of which is shown in the
cross sectional view of FIG. 2. The basic marker 10 contains an
electrical circuit which includes at least one inductor and one
capacitor which in combination resonate at at least one fundamental
frequency. Such construction may then be directly used as a marker
in an electronic article surveillance system. Although not so
shown, the construction may be further modified such as by the
addition of outer layers upon which information identifying the
user may be printed, or by the addition of a pressure-sensitive
adhesive layer to enable the marker to be affixed to the object to
be protected.
In the embodiment shown in FIGS. 1 and 2, the basic marker 10
comprises a flexible dielectric sheet 12 to opposite surfaces of
which are affixed electrically conductive metal layers, the top
most such layer is shown as element 14. The layer 14 is shown to be
configured into a number of portions to be further identified but
may be seen to be a substantially square multi-turn spiral path
forming an inductor 16 and to further have substantially triangular
areas 18 and 20 which act as capacitor pads. A like layer (unshown)
is affixed to the opposite side of the dielectric sheet 12 such
that opposing triangular areas opposite the areas 18 and 20 in
concert with the dielectric sheet sandwiched therebetween form
discrete capacitors. Such a layer may desirably also include
another multi-turn spiral inductor, the respective legs of which
are precisely positioned opposite the legs of the inductor 16 and
in combination with the dielectric sheet sandwiched therebetween
provide additional capacitance in the circuit due to distributed
capacitance effects.
The dielectric sheet 12 is preferably a film of a thermoplastic
material having desirable dielectric properties, as well as other
desirable properties, such as being readily bonded to itself and to
the metal layers 14, an outer paper covering, adhesive layers, etc.
Polyethylene sheets, for example, 0.023 millimeters thick have been
found to be particularly desirable, as such material provides a low
dissipation factor, high dielectric strength and may be directly
extruded onto a polyester carrier. The exposed face of such a
polyethylene sheet is also readily bonded to a metal foil. The
marker shown in FIG. 1 is further desirably formed from a preform
in which the polyester has affixed to only one side thereof, a
metal foil configured into two portions, one being substantially
the mirror image of the other. Such a preform may them be folded
over, leaving the two portions of the metal layer on the outer
surface opposing each other and the inner surfaces of the
polyethylene bonded together to provide a dielectric layer twice as
thick as the original sheet. If desired, the opposing metal foil
layers may be connected via connecting tabs 22 and 24 which extend
around the folded over edge.
The conductive layer 14 is preferably formed of a thin-film of a
highly conductive metal, such as copper or aluminum, and may be
configured utilizing a variety of techniques known to those skilled
in the art. Particularly, it is desirable to form the configured
pattern via standard printed circuit methods wherein an original
stencil pattern is screen printed with an etch resistant ink onto
the metal layer and the non-ink coated portions are subsequently
etched away. Other analogous techniques known to the art may
similarly be utilized.
A construction having the dimensions of approximately 2 inches (5
cm) on each side with the opposing metal foils separated by the
double thickness of the polyethylene sheet, hence a distance of
approximately 0.046 millimeters, and wherein six turns are provided
in each of the spiral portions, each leg having a width of about 1
millimeter and a spacing of about 0.75 millimeters therebetween,
has been found to provide a fundamental resonant frequency of
approximately 4.5 megahertz.
Of particular importance to the present invention is the addition
provided by the element 26 such that reversible deactivation and
reactivation of the marker is enabled. The element 26, shown in
detail in the cross-sectional view of FIG. 2, includes a
multi-layer sandwich construction wrapped around one edge of the
marker, thereby partially enclosing the dielectric sheet 12 and the
multi-turn inductors, with the ends of the wrap extending toward
the center of the marker and beyond the edge of one side of the
multi-turn inductors. In the cross-sectional view of FIG. 2, the
legs of the inductor 16 may clearly be seen as elements 28, which
elements are juxtaposed with like elements 30 of the inductor on
the opposite surface of the dielectric sheet 12. Extending
symmetrically outward from the legs 28 and 30 of the multi-turn
inductors, the element 26 first includes an insulative layer 32
which prevents the respective legs of the inductors from being
shorted out by subsequent conductive layers to be described
hereinafter. The insulative layer 32 may be formed of any
conventional insulative material, such as polyethylene or other
polymeric web, kraft paper, or the like, and may also include an
adhesive to maintain the layers in position.
Extending outward from the insulative layer 32 is layer 36, formed
of a low coercive force, high permeability material such as
permalloy or the like. As such materials are also typically
conductive, the need for the insulation layer 32 becomes apparent.
The high permeability layer 36 is positioned to provide at least
one complete magnetic path around the legs 28 and 30 of the
multi-turn inductors, with the legs extending through the closed
magnetic path. Thus, as shown in FIG. 2, the high permeability
layer 36 is wrapped around the outer edge of the inductor legs and
extends beyond the edge of the legs toward the inner portion of the
marker where the ends of the layer 36 are juxtaposed such that
magnetic coupling between the adjacent layers completes the
magnetic path.
Extending further outward from the high permeability layer 36, and
adhered thereto by pressure sensitive layer 38 are layers 40 and 42
of a relatively high coercive force, permanently magnetizable
material to provide a magnetic bias field. Such a material may
preferably be vicalloy, high silicon steel, barium ferrite,
flexible rubber bonded magnet constructions or the like. A
preferred construction includes additional alternating layers of
high permeability low-coercive force material. In the embodiment
shown in cross-section of FIG. 2, adhered via insulative, adhesive
layers 44, additional layer 46 formed of a low coercive force, high
permeability material, and 48 and 50 of high coercive force
permanently magnetizable material are thus added to improve the
efficiency of the closed magnetic path provided by the high
permeability material and the magnetic bias provided by the layers
of permanently magnetizable material.
As thus constructed, when the layers of permanently magnetizable
material 40, 42, 48 and 50 are unmagnetized, such that no magnetic
bias field is present, the localized alternating field associated
with current induced into the inductors 28 and 30 will reverse the
magnetic state of the high permeability low coercive force material
36 and 46. Resultant hysteresis losses then extract energy from the
field and lower the effective Q-factor below the predetermined
value. The tag is thus deactivated and detection is prevented.
Conversely, when the layers 40, 42, 48 and 50 of high coercive
force material are magnetized, the layers of high permeability
material are subjected to a non-varying, unidirectional bias field
more intense than that provided by the localized alternating field.
The bias field thus prevents the magnetic states in the low
coercive force material from reversing. Accordingly, hysteresis
losses do not occur, no reduction in the Q-factor is observed, and
the tag is thereby activated and detectable.
The preferred configuration shown in FIGS. 1 and 2 utilizes two
wraps of 0.6 mil (15 .mu.m) thick, quarter inch (6.3 mm) wide
strips of permalloy, with approximately one quarter of an inch (6.3
mm) extending inwardly of the innermost legs of the inductors thus
providing an overlap area of approximately one quarter inch square.
When activated by permanently magnetizing the adjacent vicalloy
sections the construction was found to be detectable at a distance
of over 39 inches from the transmitting antenna in a typical
electronic article surveillance system, such as the 3M Model ET
2000 system. In contrast, the same marker design without the
deactivating elements is typically detectable at a distance of
about 42 inches.
EXAMPELS 2-3
In order to better understand the effects of covering differing
amounts of the legs on one edge of the inductors, alternative
embodiments of that shown in FIGS. 1 and 2 were constructed wherein
the same amount of permalloy present in the two 1/4-inch wide
layers was used, but wherein that amount of material was provided
as a single folded-over layer one-half inch wide (Example 2), or as
four folded-over layers, each one-eighth of an inch wide, on top of
each other (Example 3). The marker (Example 2) containing the
single wrap one half inch wide was found to be detectable at a
distance of only approximately 30 inches and the marker having four
wraps, one eighth of an inch wide, was detectable at a distance of
approximately 41 inches. It may thus be seen that trade-offs can be
obtained, with markers having four or more layers providing
virtually identical performance as that obtained from a single
status marker, however, at the expense of more involved
manufacturing procedures than that required when fewer layers are
used.
EXAMPLE 4
In an alternative embodiment, the permanently magnetizable strips
of vicalloy shown in FIGS. 1 and 2 were replaced with strips of
AISI type 301 rolled Stainless steel, 1 mil (25 .mu.m) thick,
having the same length as the interleaved strips of permalloy, such
that they extend beyond the inner most leg of the inductors and
separated the strips of high permeability material extending toward
the center of the marker. When the stainless steel sections were
permanently magnetized, thereby preventing any change in the
magnetic state of the permalloy strips, the marker remained in a
sensitized state and was found to be detectable up to 33 inches
away from the transmitting antenna when tested as discussed above.
When the bias field was made even more intense, such as by
providing a section of magnetized barium ferite rubber bonded
magnet material adjacent the high permeability sections, a slightly
improved result was observed, i.e., that the marker was detectable
up to 35 inches away from the lattice, thus suggesting that the
stainless steel elements were not as effective as the vicalloy
sections discussed in conjunction with FIGS. 1 and 2.
EXAMPLE 5
In the embodiments discussed hereinabove in conjunction with FIGS.
1 and 2, magnetic members were folded around one edge of the marker
and extended beyond the innermost leg of the multi-turn inductors
to provide overlapping magnetic members to complete a closed
magnetic path. As set forth in FIGS. 3 and 4, a similar
construction may result without folding the magnetic members. As
may there be seen, such a marker 52 may be constructed from the
same folded over and superimposed multi-turn inductors with
capacitor pads and distributed capacitance provided by the opposed
multi-turn inductors as shown in FIG. 1. The embodiment of FIGS. 3
and 4, however, differs in that the magnetic construction 54
extends across the entire width of the marker, thereby providing
two separate closed magnetic paths which enclose opposite legs of
the multi-turn inductors. Thus as shown in detail in the
cross-sectional view of FIG. 4, in Example 5 the substrate 56 has
laminated to opposite surfaces thereof the multi-turn inductors 58
and 60 just as in the previously described embodiment. In this
embodiment, however, single 1/4-inch (6.3 mm) wide strips 62 and 64
of permalloy are bonded to the opposing surfaces of the multi-turn
inductors 58 and 60, separated therefrom by thin insulative layers
66 and 68. In a particularly preferred embodiment, the insulative
layers 66 and 68 consist of a two mil (50 .mu.m) thick layers of
type 467NBA transfer adhesive manufactured by Minnesota Mining and
Manufacturing Company. On the outside of each respective permalloy
layer 62 and 64 are positioned layers 70 and 72 of high coercive
force, permanently magnetizable material, such as vicalloy, which
layers are in turn bonded by transfer adhesive layers 74 and 76.
The permalloy layers 62 and 64 extend beyond the outer portion of
the substrate 56 and opposing outermost legs of the inductors 58
and 60 such that in the extending portion the permalloy layers are
separated only by the insulating layers 66 and 68 and are
sufficiently close together to provide a substantially closed
magnetic path. Where the permalloy layers extend inwardly beyond
the innermost legs of the inductors, the permalloy layers are
similarly close together to provide a closed magnetic path in that
region.
EXAMPLE 6
FIG. 5 shows a cross-sectional view of yet another embodiment of
the reactivatable marker of the present invention, taken across one
edge. The basic marker 78 may be seen to comprise a substrate 80 on
opposite sides of which are juxtaposed multi-turn inductors 82 and
84 as described in conjunction with FIGS. 1-4. The magnetic
construction 86 which enables alternative activation and
deactivation is electrically insulated from the inductors 82 and 84
by means of a thin insulative sheet 88. This embodiment, however,
differs from that in Examples 1-4 (FIGS. 1 and 2) in that layers of
permalloy 90 and 92 are not separated by layers of high coercive
force material. Rather, the permalloy layers are separated from
each other only by a thin adhesive layer 94. A given amount of high
permeability material of a given thickness is desirably present in
order to provide a sufficiently low reluctance magnetic path such
that hystersis losses therein will reliably deactivate the marker.
If a thinner material is used, less material is required in total.
The material is desirably provided in a plurality of thin layers,
such as sheets of permalloy, preferably 0.6-1.0 mil (15-25 .mu.m)
thick. Finally, in the embodiment shown in FIG. 5, high coercive
force, permanently magnetizable, strips 96 and 98 are adhered to
opposite surfaces of the outermost permalloy layer 92. While a
single layer 96 or 98 is shown on each side, such a layer may
similarly be composed of one or more layers of a material, such as
vicalloy or the like, in order to provide a unidirectional magnetic
field of sufficient intensity to prevent the magnetic state within
the permalloy layers from reversing.
EXAMPLE 7
Another alternative embodiment is set forth in the cross-sectional
view of FIG. 6 in which the insulating substrate 100 and multi-turn
inductors 102 and 104 are the same as in the preceding figures. As
there shown, the magnetic structure 106 is affixed to opposite
sides of one edge of the marker, and extends beyond the innermost
and outermost layers of the inductors to complete the closed
magnetic path. Thus as shown in FIG. 6, two layers 108 and 110 of
high permeability, low coercive force material are provided on one
side of the multi-turn inductor 102 while two additional layers 112
and 114 are provided on the opposite side opposing the multi-turn
inductor 104. As before, a thin insulating layer is provided
between the inductors and the high permeability layers. Likewise,
the entire magnetic construction 106 is bonded together by thin
insulating adhesive layers interleaved between the respective high
permeability layers 108, 110, 112 and 114 are layers 116, 118, 120
and 122 of a high permeability permanently magnetizable material
such as vicalloy. Such a construction has been found to have a
performance substantially the same as that of the embodiment shown
in FIGS. 1 and 2, but may be preferred in some applications due to
the ease of manufacture.
EXAMPLE 8
Another alternative marker 124 is shown in FIG. 7 to comprise an
insulative substrate 126 having on opposed surfaces thereof a
multi-turn inductor 128 and 130 as in the preceding figures. Layers
132 and 134 and 136 and 138 of high permeability material are
provided on the outer surfaces of the multi-turn inductors 128 and
130. Interposed between the respective high permeability layers 132
and 134 or 136 and 138 are layers of high coercive force,
permanently magnetizable, material 140 and 142. Likewise, the
entire marker 124 is bonded together by thin insulating adhesive
layers (not shown). This embodiment thus differs from that of
Example 7 (FIG. 6) in that only one layer of permanently
magnetizable material is provided on each side of the marker.
It may thus be recognized that a number of alternative
constructions may be provided, it being relatively immaterial
whether the permanently magnetizable sections are only present as
the outermost layer as in FIG. 5, are interleaved between opposing
layers of the construction as in FIG. 6, or are only provided
within the construction as in FIG. 7. The only limitation to the
location of the permanently magnetizable material is that it should
be located as close to the low coercivity material as practical to
provide for an efficient magnetic coupling between the permanently
magnetizable material and the low coercivity material.
EXAMPLES 9 AND 10
The embodiments described hereinabove have all been constructed
utilizing sheets of permalloy as the high permability, low coercive
force material. Other high permability materials may similarly be
used in the present invention.
In Example 9, the folded-over permalloy layers shown in FIGS. 1 and
2 were replaced with approximately 0.001 inch (25 .mu.m) thick,
3/16" (4.8 mm).times.7/16" (1.1 cm) folded over layers of
"Silectron" alloy provided by Arnold Engineering. Depending upon
the number of layers of such material included, the respective
markers were found to be desensitizable to substantially the same
degree as with permalloy. Similarly, in Example 10, varying numbers
of layers of YEP-HD, a high permeability alloy available from
Hitachi Metals Ltd., and having the composition of Fe:12.4%;
Mo:2.1%; Nb and Ti:4.2%, balance Ni, were substituted for the
permalloy layers. Again, substantially the same degree of
sensitivity and desensitizability was observed.
Similarly, the addition of various high coercive force, permanently
magnetizable materials, was found satisfactory to enable the thus
modified markers to be reactivated upon permanently magnetizing
such materials.
EXAMPLES 11-14
Depending upon the Q-factor of the resonant circuit present in the
underlying marker, varying amounts of high permeability material
may desirably be provided to ensure complete deactivation. In
example 11, multi-turn inductors formed of etched copper foil,
rather than of aluminum, typically having a very high Q-factor,
were found to require more high permeability material to be present
in order that hysteresis losses will lower the Q-factor
sufficiently to inhibit detection. Such a marker, configured as
shown in FIG. 1, but in an unaltered state and evaluated in a
system such as that identified above, was found to be detectable 49
inches from the transmitting antenna. The effect of two wraps of
one mil permalloy of varying widths wrapped about one edge of such
a marker as shown in FIG. 2 was as follows:
______________________________________ Width of Range of Detection
(inches) 1 mil Desensitized permalloy (no bias Sensitized Example
(2 wraps) field) (bias field)
______________________________________ 11 0 49 49 12 3/16 5 40 13
4/16 0 45 14 5/16 0 38 ______________________________________
It will thus be recognized that if too little high permeability
material is present, complete desensitization cannot be obtained,
while if too much material is present, the range at which the
sensitized marker can be detected is deleteriously affected,
without any benefit in the desensitization aspects.
The relative amount of high permeability material, such as
permalloy, desirably provided to reliably desensitize a marker may
also be evidenced from the following tests with an RF detection
system adjusted to provide a less sensitive detection, i.e., to be
capable of detecting an unmodified basic marker as in Example 1,
forty inches away from the transmitting antenna as opposed to
detectability forty-two inches away when the system sensitivity is
increased. When such a marker was modified to have a single layer
of 3/16 inch wide, one mil strip of permalloy wrapped entirely
around one leg of the multi-turn inductor, the modified tag was
still detectable in its desensitized state 71/2 inches the antenna,
thus indicating that an insufficient amount of permalloy was yet
present. A still less acceptable configuration resulted when either
one or two layers of one mil by 1 inch long strips of 3/16th wide
permalloy were provided on only one side of the marker. Such a
configuration resulted in the marker still being detectable at
distances in the order of 25 inches away from the antenna. In
contrast, when such 3/16th inch by one inch pieces were placed on
each side of the marker with the edges extending beyond the leg of
the inductor in substantial magnetic contact with each other, thus
completing a closed magnetic path, the marker was only detectable
at about 13 inches away from the antenna. Still more preferred,
when two such 3/16th inch by one inch long strips were positioned
on each side of the marker, it was only detectable immediately
adjacent one corner of the antenna. When still more material was
provided, such as by wrapping two layers, 3/16th of an inch wide by
2 inches long, around one edge of the marker thus providing two
layers on both sides of the inductor leg, the marker was totally
undetectable.
EXAMPLE 15
The high permeability materials such as permalloy used to provide
desensitization as in the preceding Examples, may also be replaced
by analogous non-crystalline, amorphous materials. In Example 15,
the same size strip as used in Example 1, but formed of an
amorphous metal, Type 6025, available from Vacuumschmelze GmBH, in
an unannealed, as-cast-condition, having a composition of
approximately 66 atomic percent cobalt, 40 percent iron, 15 percent
silicon, 13 percent boron and 2 percent molybdenum, was observed to
be completely undetectable and hence slightly better in performance
than the above noted construction of permalloy. Indeed, when only
one layer of such a same shape and same material was used, the
performance was only marginally worse than that observed for two
layers of permalloy. Furthermore, when the amorphous material was
annealed, thereby increasing its permeability to enhance its
properties for certain applications, but wherein a somewhat higher
coercive force resulted, a marker having two such folded over
layers of the annealed material was found to be detectable 13
inches away from the transmitting antenna. Similarly, a marker
having three such folded over layers was still detectable 10 inches
away from the antenna. These results demonstrate that while it is
desirable to have a high permeability material, it is perhaps
equally important that the material have very low coercive force
such that its magnetic state can be readily altered by the low
localized fields induced into the marker when present in an
interrogation zone.
In further tests of the efficacy of amorphous, low coercive force
materials, markers modified with strips of various compositions
obtained from Allied Corporation were compared with markers
modified with permalloy and the Type 6025 alloy. In Examples 16-18,
strips of Type 2826MB2DG (down grain) unannealed (Ex. 16) and
strips of 2705MDG (down grain) in the unannealed state (Ex. 17) and
CG (cross grain) in an annealed state (Ex. 18) were tested. Under
the testing conditions, the unaltered markers were detectable
approximately 45 inches away from the transmitting antenna. One
layer of all of the various materials, having a constant
cross-section of 0.0001875 square inches, (for example, 1 mil
thick.times.3/16 inches wide) was simply placed over one edge of
the marker, without providing a closed magnetic path. A single
layer of permalloy was found to desensitize the marker only to the
extent that it could still be detected approximately 12 inches away
from the transmitting antenna. The marker having the 6025 type
alloy was desensitized to somewhat greater extent, being detectable
only approximately 10 inches from the transmitting antenna, and the
markers of Example 16-18 were desensitized to a slightly less
extent, such tags still being detectable at distances in excess of
2 inches from the transmitting antenna.
When the above materials were provided with a closed magnetic path
by providing strips of the same cross-sections but folded over as
in Example 1, the permalloy modified marker was still detectable at
a distance of 1.75 inches from the transmitting antenna. The marker
modified with high cobalt amorphous material (Type 6025) was not
detectable under any conditions, the markers modified with the
2705M compositions were still detectable at distances of 5 to 10
inches away from the transmitting antenna, and the marker modified
with the 2826MB2 composition was detectable at a distance of 18
inches. When additional amounts of such materials were provided,
such as by folding over two or more overlapping strips, the
resulting markers were totally desensitized.
As noted above, in a preferred construction of the present
invention, the high permeability material is provided in a
relatively thin strip, with plurality of such strips being
sandwiched together.
For example, two 3/16 inch (0.48 cm) wide strips of permalloy
folded over one edge of a marker as in Example 1, each strip being
2.5.times.10.sup.-4 inches (6.3.mu.m) thick have been found to be
approximately equally effective as four strips of the same length
and width, each being 1.25.times.10.sup.-4 inches (3.17.mu.m) thick
(i.e., the same total thickness of 5.times.10.sup.-4 inches
(12.7.mu.m) in both cases), both combinations being capable of
fully desensitizing a marker. In contrast, two layers each
5.times.10.sup.-4 inches (12.7 .mu.m) thick and three layers each
10.times.10.sup.-4 inches (25.4 .mu.m) thick have been found
necessary to achieve full desensitization.
As the use of relatively thin sheets of high permeability material
are thus desirably used, while at the same time necessarily
including sufficient magnetic material to enable full
desensitization, it has been determined that such a quantity of
material can be provided in a variety of configurations. Thus, for
example, such a quantity of material may be provided as one or more
strips adjacent each other, extending over a substantial portion of
the multi-turn inductor rather than as a laminate. However, the
laminated configuration has been found to be preferred. While
little difference in the various configurations result from the
amount of material necessary to desensitize the marker, when the
marker is sensitized by the presence of permanently magnetized
material, the resensitized marker formed of laminated narrow strips
is detectable at greater distances.
EXAMPLE 19
The present invention, while described hereinabove with regard to a
printed circuit type marker, is similarly useful in a variety of
other marker configurations. Thus, for example, as shown in FIG. 8,
a basic marker 144 may comprise a resonant circuit formed of an
inductor 146 formed of a plurality of turns of insulated
transformer wire, having secured to opposite ends thereof a
capacitor chip 148. The marker 144 may similarly be modified to
enable reversible sensitization and desensitization by a magnetic
structure 150, as discussed hereinabove. Thus, while in Example 19,
one or more complete wraps of a high permeability material 152
between adjacent layers of which are positioned small rectangular
sections of a high coercive force permanently magnetizable material
154, were used, the other analogous configurations are also
useful.
EXAMPLE 20
In similar fashion, FIG. 9 sets forth a plane view of another
configuration of a basic RF marker 156 wherein a printed circuit
multi-turn spiral inductor 158 is formed via conventional etching
techniques in a conductive sheet applied to one surface of a
dielectric substrate 160. In this marker, the multi-turn inductor
terminates at relatively large discrete areas 162 and 164. A
conductive sheet on the opposite surface of the dielectric sheet
160 is configured with opposing conductive areas 166 and 168 to
provide opposing capacitor pads. In the embodiment shown in FIG. 9,
the dielectric sheet 160 has been cut away in the area 170 to
provide an opening therethrough, allowing the magnetic construction
172 to be wrapped completely around one leg of the multi-turn
spiral 158 in the same manner as shown in FIG. 8.
A cross-sectional view of the structure 172 is set forth in FIG.
10, where it may be seen that the basic marker construction
includes a dielectric substrate 174 having affixed to one surface
thereof the multi-turn inductor 176. To prevent the magnetic
structure from shorting out the inductor 176, a layer of electrical
insulation 178, such as conventional kraft paper or the like, is
first wound around the insulator, and the insulation is in turn
covered by one or more wraps 180 of high permeability material as
described above. Finally, one or more layers of a high coercivity
permanently magnetizable material 182 are supplied. The entire
construction may be enclosed within a protective outer coating 184,
and pressure-sensitive adhesive layers added to enable the marker
to be affixed to articles to be protected. The adhesive layers may
in turn be temporarily covered with a low adhesive release liner
186.
EXAMPLE 21
Yet another alternative construction is set forth in the plane view
of FIG. 11, where the basic marker 188 may be seen to be formed of
metal foil spirals on opposite sides of a dielectric sheet, one of
which spiral 190 may be seen. Such a configuration may readily be
provided by a die-stamping technique as set forth in U.S. Pat. No.
4,482,874. As the portion of the dielectric web on the interior of
the multi-turn spirals may be desirably removed during the
die-stamping operation, the magnetic structure 192 as discussed may
readily be wound about one leg of the multi-turn spiral to provide
the alternate sensitization/desensitization capability.
EXAMPLE 22
In addition to the embodiments discussed hereinabove, wherein a
single strip of high permeability material is folded or wrapped
about all of the turns along one leg of the multi-turn inductor, it
is also within the scope of the present invention that each turn of
the multi-turn inductor have folded or wrapped about it a separate
strip of high permeability material. In such a configuration, a
separate strip of high coercive force material may also then be
wrapped about each individual strip of high permeability material.
Alternatively, a single strip of such high coercive force material
may be positioned adjacent to all of the fold or wraps of high
permeability material.
In Example 22, set forth in FIGS. 12 and 13, an alternative marker
191 is shown to comprise a dielectric substrate 193 having formed
on one surface thereof a multi-turn inductor 194. As shown in more
detail in the cross-sectional view of FIG. 13, the marker 191
includes only a single, multi-turn inductor on one side of the
dielectric substrate 193, with the remainder of the resonant
circuit being formed by a discrete capacitor not shown. The
sensitizer/desensitizer assembly 196 may be seen in the
cross-sectional view of FIG. 13 to comprise layers 198 of high
permeability material which completely surround each turn of the
multi-turn inductor 194. On the opposite surface of the dielectric
substrate 193 is affixed a single strip of high coercive force
material 200. The layers 198 of high permeability material are
conveniently provided by successive plating techniques. Such layers
may be formed by first providing on a suitable dielectric sheet a
first sheet of high permeability material, such as an iron-nickel
alloy, having thereover a high conductivity metal layer such as
copper or aluminum. The double layer laminated may then be etched
to form the multi-turn inductor, with a layer of high permeability
material remaining under each conductive strip forming the multiple
turns.
Subsequently, a similar high permeability material may be added via
vapor deposition, electro-plating or similar techniques to form the
top and side portions of the layer 198, thereby completing an
enclosed magnetic path.
EXAMPLE 23
An alternative embodiment to that shown in FIG. 13 is shown in FIG.
14 where a plated assembly is shown to include a dielectric
substrate 202 having on a surface thereof the following successive
layers: a high permeability material 204, a high conductivity
material 206, a second high permeability layer 208 and an uppermost
layer 210 of high coercive force material. The multi-layer laminate
may then be etched using conventional techniques to form a
multi-turn spiral thereby removing unwanted portions of all four
layers. Subsequently, a completed magnetic path may be provided by
plating or vapor depositing high permeability material 212 on both
sides of the remaining multi-turn portions.
It is also within the scope of the present invention that the means
for controllably magnetically biasing the high permeability
material may be other than a piece of high coercive force
permanently magnetizable material which is permanently affixed
adjacent the high permeability material. Thus, for example, as
shown in the partial cross-sectional view of FIG. 15, a marker 214
may comprise a dielectric substrate 216 having on opposite surfaces
thereof opposing multi-turn inductive spirals 218 and 220, with the
opposing conductive paths forming a distributed capacitor which
completes the resonant circuit. On opposite surfaces of the marker
214 are provided strips 222 and 224 of high permeability material
which enclose a portion of one leg of the multi-turn inductors 218
and 220. A portion of the high permeability strips 222 and 224
extends beyond the end of the dielectric substrate 216 so as to be
magnetically coupled together to complete a closed magnetic path.
However, rather than the layers 222 and 224 being as close together
in the overlapping region as possible, the layers are permanently
separated to allow a section of high coercive force material 226 to
be removably inserted within the gap. Thus, when the marker is
sensitized, the permanently magnetized section 226 will be inserted
within the gap. The bias magnetic field thereby established
prevents alteration in the magnetic states of the high permeability
strips 222 and 224 such that no hysteresis loss occurs, no
reduction in the Q of the marker occurs and the marker is thereby
detectable. Alternatively, when the marker is desensitized, the
piece 226 is removed, thereby allowing magnetic coupling between
the opposed strips of high permeability material 222 and 224. The
closed magnetic path then allows alteration in the magnetic states
of those strips 222 and 224, and hysteresis losses reduce the Q of
the marker below the point of detectability. It will also be
appreciated that a removable magnet for sensitization does not have
to have a configuration allowing it to fit into a gap. Such a
magnet could be located anywhere where there is room for it in the
proximity of the high permeability material such that it saturates
that material sufficiently to sensitize the marker.
In an alternative embodiment it has also been found that a
desensitizable/sensitizable marker may be provided by providing
sections of ferrite materials adjacent at least one leg of the
multi-turn inductors. Thus, for example, a thin sheet of low
coercivity ferrite sufficiently large to overlap the entire
multi-turn inductive spiral has been found sufficient to completely
desensitize the marker and may yet be made resensitizable by
positioning adjacent thereto a similarly large sheet of a permanent
magnetic material such as a rubber-bonded barium ferrite. As such a
ferrite is non-conductive, no apparent shielding or deleterious
effects on the Q of the circuit was detected. Such a configuration,
however, may be less desirable than the embodiments discussed above
as a ferrite sheet is more expensive than a similarly sufficient
amount of permalloy. A preferred construction wherein ferrites are
used, is to utilize two small chips of ferrite on opposite sides of
one leg of the multi-turn spiral in a manner more analogous to that
provided by the wrap of permalloy. When such a configuration of
chips approximately 3/8th of an inch square was utilized, the
marker was completely desensitized and was detectable at over 41
inches when sensitized with a section of permanently magnetized
rubber-bonded ferrite material positioned adjacent one side of the
marker.
A three dimensional view of the system of the present invention
wherein the desensitizable/sensitizable marker is desirably used,
is shown in the perspective view of FIG. 16. As may there be seen,
the system 230 is desirably installed to define an interrogation
zone 232 such as in an exit-way defined by adjacent walls 234 and
236. Positioned adjacent the wall 234 is an antenna panel 237,
within which are positioned transmitting and receiving antennas.
The antennas are coupled to a transmitter and receiver circuit 238
and 240, respectively. Such a transmitter circuit 238 will generate
electromagnetic signals which are transmitted via the transmitting
antenna within the panel 238 to generate a radio frequency field
within the interrogation zone 232. Upon passage within the zone of
an object 242 to which is affixed a marker 244 according to the
present invention, the marker, when in its sensitized state, will
be excited to produce oscillations at its resonant frequency. Those
oscillations will be detected within the receiver antenna within
the receiver panel 237, and the received oscillations coupled to
the receiver circuit 240, wherein detector circuitry will
distinguish the received signals from other electromagnetic noise
and produce an alarm signal when appropriate. A
desensitization/sensitization apparatus 246 may be conveniently
positioned adjacent the interrogation zone, such as at a checkout
counter. The sensitization-desensitization apparatus 246 preferably
comprises means for establishing a magnetic field controlled to be
either unidirectional or bidirectional. If it is desired to
magnetize the permanent magnetic member included within the marker,
to thereby sensitize it, a unidirectional field is produced by the
apparatus 246. Conversely, if it is desired to desensitize the
markers, an alternating magnetic field is produced by the apparatus
246, such that as the marker is gradually removed from the
apparatus 246, the permanent magnetizable member of the marker will
be exposed to an alternating field of gradually decreasing
intensity, such that it ultimately is left in a demagnetized state.
Alternatively, as is well known to those skilled in the art, the
alternating magnetic field may gradually decrease in intensity, and
leave the magnetizable element in a demagnetized condition.
* * * * *